syms b11 b12 b21 b22
A=[3 -6; -2 4];
B=[b11 b12; b21 b22];
A*B

A=[3 0 -6 0; 0 3 0 -6; -2 0 4 0; 0 -2 0 4]
A(1,:)=A(1,:)/A(1,1);
A(3,:)=A(3,:)-A(3,1)*A(1,:);
A(2,:)=A(2,:)/A(2,2);
A(4,:)=A(4,:)-A(4,2)*A(2,:);
A